Injection system and associated operating method

ABSTRACT

A feeding system for an absorber liquid containing a neutron poison, in particular for a quick shut-down of a nuclear reactor, has a storage container for the absorber liquid and is configured for high operational reliability with simple construction. In particular, a chemical decomposition of the absorber liquid or corrosion of the container wall of the storage container is to be excluded. For this purpose, the storage container is connected to a pressure container via an overflow line, wherein the pressure container is filled with a motive fluid.

CROSS-REFERENCE TO RELATED APPLICATION

This is a continuation, under 35 U.S.C. §120, of copending internationalapplication No. PCT/EP2006/011097, filed Nov. 20, 2006, which designatedthe United States; this application also claims the priority, under 35U.S.C. §119, of German patent application No. DE 10 2005 057 249.9,filed Nov. 29, 2005; the prior applications are herewith incorporated byreference in their entirety.

BACKGROUND OF THE INVENTION Field of the Invention

The invention relates to an injection system for a neutron-poisoncontaining an absorber liquid, in particular for the emergency shutdownof a nuclear reactor, containing a reservoir vessel for the absorberliquid. The invention furthermore relates to a method for making anabsorber liquid which is under an operating pressure available in suchan injection system. The invention additionally relates to a method forinjecting the absorber liquid into a component, which is connecteddownstream of the reservoir vessel, of a plant, for example into anuclear reactor. Finally, the invention relates to a nuclear powerplant, in particular a boiling-water nuclear power plant containing aninjection system of this type.

In nuclear engineering plants, generally an injection system for aso-called absorber liquid is provided as a safety-relevant device. Inparticular in a boiling-water nuclear power plant it is necessary tomake available rapidly acting measures for the emergency shutdown of thereactor core, if for example the drive of the control rods, which areused to control the nuclear reaction in the normal case, fails. For thispurpose, introduction of an absorber liquid with a high absorption crosssection for neutrons may be provided in the case of an incident.Usually, a boron solution is used for this purpose, wherein the boron,which is in this context also referred to as a neutron poison, effectsthe absorption of free neutrons. In this manner, the reactor core is, inan incident situation, safely converted to a subcritical state.

By making available the absorber liquid in a reservoir vessel of theinjection system at a high pressure, quick initiation of the injectionprocess into the reactor core is possible at all times without the needto first activate active system components which are prone to failure,such as conveying pumps. In order to achieve this passive safetyconcept, it is therefore necessary to make available the absorber liquidpossibly for years at a comparatively high operating pressure.

In accordance with a known concept, the pressure in a reservoir vessel,which is in the form of a pressure accumulator, for the absorber liquidcould build up by way of a nitrogen cushion which is located above theliquid. To this end however a complex nitrogen distribution system isnecessary. A disadvantage in the case of the high operating pressurewhich is envisaged according to the design is also the relatively highspace requirement for the nitrogen cushion relative to the volume of theabsorber liquid. Furthermore, over time the nitrogen dissolves at leastpartially in the absorber liquid (usually boric acid), with the resultthat, when liquid is injected, a non-condensable gas is also introducedinto the reactor, which among others negatively affects the coolingeffect of condensers or emergency condensers.

Published, non-prosecuted German patent application DE 198 46 459 A1,corresponding to U.S. Pat. No. 6,895,068, discloses an injection systemfor cooling liquid for the emergency cooling of a nuclear reactor, whichachieves the necessary operating pressure by heating the cooling liquid,which is stored in a pressure accumulator vessel, using a heatingapparatus arranged in the pressure accumulator vessel. In the process, avapor cushion is formed over the liquid level by evaporating the liquidas a function of the original filling height. If it is needed, that isto say in a reactor incident situation, the vapor cushion pushes thecooling liquid, with simultaneous relief, into the reactor core througha supply line which is connected in the bottom region of the pressurevessel. Arranging the heating apparatus in an upper section of theaccumulator vessel effects a temperature layering of the cooling liquid,with the result that, if needed, first comparatively cold and laterincreasingly hot cooling liquid flows out of the accumulator vessel. Theapplication of this concept in the context of a boron injection systemin which a boron-containing liquid is used not only for cooling purposesbut primarily for the emergency shutdown of a nuclear reactor islikewise known.

Chemical tests have now concluded, however, that if the absorber liquid,which can advantageously be in the form of a boron solution, is storedfor a number of years at the temperatures which are necessary togenerate pressure and which are envisaged according to the design withinthe context of the described concept, a progressive chemicaldissociation of the absorber liquid must be expected. Additionally anincreased interaction between the absorber liquid and the material ofthe vessel walls could occur, which under certain circumstances has adisadvantageous effect on the pressure stability or the leak tightnessof the reservoir vessel.

SUMMARY OF THE INVENTION

It is accordingly an object of the invention to provide an injectionsystem and an associated operating method which overcomes theabove-mentioned disadvantages of the prior art devices and methods ofthis general type, which injection system ensures, while keeping theconfiguration simple, permanently high operational reliability andavoids the stated disadvantages of the known systems. Furthermore, amethod which is particularly suitable for operating the injection systemwill be specified—to be precise in each case for the storage period andfor the actual injection process.

With the foregoing and other objects in view there is provided, inaccordance with the invention, an injection system for a neutron-poisonfor an emergency shutdown of a nuclear reactor. The injection systemcontains an absorber liquid, a pressure vessel filled with a propellingfluid, an overflow line, and a reservoir vessel for the absorber liquid.The reservoir vessel is connected, via the overflow line, to thepressure vessel.

With respect to the injection system, the stated object is achievedaccording to the invention by virtue of the fact that the reservoirvessel is connected, via an overflow line, to a pressure vessel, withthe pressure vessel being filled with a propelling fluid.

The invention is here based on the idea that in order to achieve highoperational reliability, chemical decomposition of the absorber liquidor other chemical reactions within the absorber liquid or with the wallsof the surrounding reservoir vessel and thus possible corrosion of thewall materials should also largely be avoided if the absorber liquid isstored for a relatively long time over a period of several years or evendecades. However, since chemical reactivity generally likewise increaseswith an increase in temperature, the absorber liquid should be stored ina comparatively cool manner, that is to say approximately at roomtemperature. On the other hand, the absorber liquid in the reservoirvessel should be able to be kept at a prespecified operating pressurefor the entire storage period, so that if it is needed, it can beinjected into a reactor cooling system or into a reactor core as quicklyand completely as possible. The measures which are necessary to generatepressure should here lead to as little influence on the absorber liquidwith respect to its other physical or chemical characteristics aspossible. This is achieved in the present case by virtue of a pressurevessel with a propelling fluid which is under operating pressure, whichpressure vessel is physically separate from the reservoir vessel and isconnected to the reservoir vessel via an overflow line which effectspressure equalization. Therefore on the one hand, due to the overflowline, the pressure in the two vessels is always the same, and on theother hand, the physical separation of the two functions “storage of theabsorber liquid” and “maintaining a pressure cushion” using a propellingfluid has the consequence that either of the two vessels can be matchedand configured, with respect to its chemical compatibility or othercharacteristics, specifically to the liquid (absorber liquid orpropelling fluid) it contains. The interactive influence between theabsorber liquid and the propelling fluid is here extremely low due tothe physical separation.

Advantageously, the pressure vessel of the injection system has aheating device. In a preferred embodiment, the heating device can beregulated and is configured with respect to its heating output forgenerating and maintaining an adjustable pressure, wherein preferablythe pressure represents the reference variable of the regulation. Theactual pressure generation can thus be effected analogously to knownsystems by virtue of the fact that the pressure vessel is filled up to aprespecifiable filling height (i.e. not completely) with a propellingfluid. Some of the propelling fluid is evaporated by the heating deviceso that a vapor cushion is formed in the upper region of the pressurevessel and the pressure is maintained by the vapor cushion. Theadvantage of the vapor cushion is that the operating pressure can veryeasily be adjusted to, and also maintained at, a desired value, forexample the saturated vapor pressure of the propelling fluid. Since thevapor is compressible, a slight temperature increase does not lead to anoverproportional pressure increase, as inevitably occurs in a pressureaccumulator vessel, for example, which is filled completely with anincompressible liquid. Overall, a known and already proven technologytogether with the associated know-how can therefore be used for thepressure generation, by which the emergency injection system accordingto the novel concept can be realized particularly simply andcost-effectively. In particular, if the overflow line, which will bedescribed in more detail below, is configured and dimensionedaccordingly, a largely thermal decoupling of reservoir vessel andpressure vessel is ensured with the result that the propelling fluid canalso be stored under an operating temperature which is high as comparedto the absorber liquid, to the extent that this is expedient for simplegeneration and maintenance or regulation of the operating pressure, ordesired for reasons of other considerations.

In a preferred development, the overflow line, which is provided forpressure equalization and for guidance of the propelling fluid, connectsthe bottom region of the pressure vessel to the ceiling region of thereservoir vessel. In other words, the connector, which is provided atthe pressure vessel, of the overflow line is arranged in a lower wallsection near the vessel bottom. The other end of the overflow lineissues on the side of the reservoir vessel in an upper section of thevessel wall. In particular, this connector can also be guided through aceiling of the reservoir vessel, which is in the form of a dome.Preferably, the overflow line has in the region of its end, which facesthe pressure vessel, a lower partial section which is guided in themanner of a siphon and has a low point which lies below the bottom ofthe pressure vessel. That end of the overflow line which is connected tothe pressure vessel, that is to say that end which is located at theentrance with respect to the direction of flow during the injectionprocess, is therefore in the form of a downpipe piece near theconnector. At the (exit) side, which faces the reservoir vessel, theoverflow line preferably has an upper partial section with a high pointwhich is arranged above the ceiling of the reservoir vessel, with theresult that here on the connector side a downpipe piece is likewiseprovided. The lower and upper partial sections of the overflow line arepreferably connected to one another by a nearly vertical riser piece.

This type of line guidance is extremely advantageous for the alreadymentioned thermal decoupling of the fluids which are stored in thepressure vessel and in the reservoir vessel, respectively, since inparticular the lower siphon-type partial section is used to largelysuppress convective heat flow or heat conduction to the reservoir vessel(in particular due to the downpipe pieces). The line arrangement alsohas the advantage that when the injection system is activated, firstcooler, liquid propelling fluid from the bottom region of the pressurevessel, later hot, liquid propelling fluid and only then vaporouspropelling fluid, which originally forms the vapor cushion in theceiling region of the pressure vessel, flows over into the reservoirvessel for the absorber liquid. In this manner, the walls of thereservoir vessel are comparatively gently preheated when the propellingfluid enters. This reduces the condensation of the propelling fluidvapor, which would otherwise very quickly heat the previously cold wallsof the reservoir vessel. A temperature shock is thus avoided.Additionally, the vapor pressure decreases comparatively slowly duringthe injection procedure due to the preheating effect caused by theliquid propelling fluid. Even after a large part of the absorber liquidhas already been introduced into the reactor, sufficient residualpressure is therefore left to completely displace the absorber liquidfrom the reservoir vessel.

Preferably, an outflow opening provided for the on-demand removal ofabsorber liquid, for example in the form of an outlet connectionprovided with a shut-off valve, is arranged in the bottom region of thereservoir vessel. The exit of the absorber liquid is here supported bythe hydrostatic pressure of the liquid column in the reservoir vessel.

In a preferred embodiment, the pressure-loaded components of theinjection system, that is to say in particular the pressure vessel, thereservoir vessel and the overflow line, are configured for an operatingpressure of more than 100 bar, in particular for an operating pressureof approximately 150 bar. This corresponds to a pressure value whichincludes sufficient contingency reserves and is particularly expedientfor use as emergency injection system in a nuclear power plant.

In order to generate and maintain the desired operating pressure whichpreferably corresponds to the saturated vapor pressure of the propellingfluid, the propelling fluid is heated, with matching to the providedgeometry of the pressure vessel, to an operating temperature of over300° C., in particular to about 340° C. In another advantageousrefinement, the pressure vessel is therefore also configured in terms ofits material selection for a permanent provision of pressurizedpropelling fluid with such a high temperature.

The reservoir vessel for the absorber liquid does not need to fulfillany particular requirements in terms of temperature resistance in theregular case, i.e. during the storage period or the standby time until areactor incident, since the absorber liquid is merely at approximatelyroom temperature. The overflowing hot propelling fluid causes acomparatively short temperature load only during the actual injectionprocedure. The reservoir vessel can therefore be made of a materialwhich is of a lower quality with respect to its heat resistance than thepressure vessel and is thus cheaper.

The injection system is, due to its design principles (in particularpassivity and longevity), suitable particularly for making available anabsorber liquid for on-demand emergency shutdown of a nuclear powerplant. To this end, the reservoir vessel of the injection system ispreferably dimensioned such that it can hold a quantity of absorberliquid which is sufficient for shutting down a nuclear reactor. Thepressure vessel is preferably dimensioned in this case such that it canhold a quantity of propelling fluid which is sufficient to completelydisplace the absorber liquid from the reservoir vessel, with thisquantity being dependent inter alia on the desired operating pressureand the desired operating temperature and on the type of fluids.

An aqueous boron solution has proven particularly suitable as theabsorber liquid for emergency shutdown and/or emergency cooling of anuclear reactor. In particular, an approximately 13% boron solution, forexample a sodium pentaborate solution, can be provided as the absorberliquid. As opposed to other conceivable absorber liquids with a highabsorption cross section for neutrons, a boron solution is distinguishedat least at temperatures which are not too high by a long shelf life andrelatively good chemical compatibility with respect to the walls of thereservoir vessel, which are usually made of steel.

Water is preferably provided as an easy-to-store propelling fluid whichhas particularly good compatibility with the boron solution. In theevaporated form, i.e. in the vapor cushion of the pressure vessel, thewater is present as water vapor.

The object relating to the method for making available an absorberliquid under an operating pressure is achieved by virtue of the factthat the operating pressure is generated by heating the propelling fluidin the pressure vessel of the injection system. The propelling fluid ishere advantageously stored in a lower region of the pressure vessel inliquid form. The action of the heating device which can preferably be inthe form of an electric heating device or of a heat exchanger systemcauses some of the quantity of liquid to evaporate, so that a vaporcushion is formed in the upper region of the pressure vessel. In aparticularly preferred refinement, the heating device is a constituentpart of a regulating system and is additionally dimensioned with respectto its achievable heating output to be sufficiently large so that avapor cushion with an adjustable, temporally constant pressure can bepermanently maintained. If a vapor cushion is present, the adjustmentand maintenance of the desired pressure value can be effectedsignificantly more easily in comparison with a pressure accumulatorwhich is completely filled with liquid.

In case of necessity, the absorber liquid which is made available underan operating pressure is injected into a component, which is connecteddownstream of the reservoir vessel, of a plant, in particular into anuclear reactor. Such a case of necessity exists for example if thecontrol elements or control rods, which are normally provided to controlthe neutron flow, cannot be inserted into the core due to a fault in thedrive or in the actuation. In this case, a valve or a shut-off apparatusof the reservoir vessel is opened, so that the pressurized absorberliquid is introduced via a connection line into the reactor pressurevessel. Preferably, due to the relief of the vapor cushion, which isformed from evaporated propelling fluid, in the pressure vessel, firstliquid and then vaporous propelling fluid flows into the reservoirvessel, wherein the absorber liquid which was originally present thereinis more and more displaced. To the extent that water or water vapor isused as the propelling fluid, thus first preferably hot water, thensaturated water and finally saturated vapor flows from the pressurevessel into the reservoir vessel, with the vessel pressure decreasing atthe same time. A temperature shock on the walls of the reservoir vesselis avoided by this advantageous sequence.

In another advantageous development, the overflow speed of thepropelling fluid during such an injection process is adjusted such thatalthough on the one hand as high a throughput per unit time as possibleis achieved and thus the reactor can be shut down quickly, on the otherhand mixing of the propelling fluid with the absorber liquid issubstantially prevented. Such a value of the overflow speed can beadjusted for instance by way of a suitable throttle device or can bepredefined already by way of the dimensioning of the overflow lineitself. The advantage of the injection method carried out in this manneris that the temperature layering, which is established due to thedensity differences in the reservoir vessel, of cold absorber liquidand, above the latter, hot propelling liquid remains. Therefore onlycold absorber liquid is introduced into the reactor core. Once theabsorber liquid is fully displaced from the reservoir vessel, thepressure in the injection system has advantageously decreased sostrongly that the flow processes stop automatically. In this way, thehot propelling fluid is kept away from the reactor core.

Expediently, the injection system is a constituent part of a nuclearpower plant. It is preferably in the form of a so-called poisoninjection system which can be used to shut down the nuclear reaction ina boiling-water reactor if, in the case of a serious incident situation,the control rods can no longer be inserted into the reactor core.Alternatively, or additionally, provision could be made for the fluidwhich is made available at high pressure by the injection system to beused for the emergency driving of the control rods themselves, by way ofwhich a hydraulic drive system is realized which is redundant withrespect to the usually electric driving of the control rods. The liquidstored in the reservoir vessel of the injection system should in thiscase therefore be regarded as drive liquid for the control rods.

In another alternative, the injection fluid is supplied in an expedientmanner to an emergency cooling system of a pressurized-water nuclearpower plant as emergency cooling fluid (usually emergency coolingwater). The injection system is therefore preferably used as a so-calledaccumulator for the emergency cooling water in a pressurized-waternuclear power plant.

The advantages achieved with the invention are in particular that, byphysically separating the functions “liquid storage” and “pressuregeneration” in the form of a reservoir vessel and a pressure vesselwhich are in each case provided for holding purposes, an effectivechemical and thermal decoupling of the absorber liquid from thepropelling fluid can be achieved, wherein the overflow line whichconnects the two components of the injection system ensures that theabsorber liquid is stored at operating pressure such that it can beremoved at any time. Such a configuration of an injection system, inwhich in particular heating of the reservoir vessel for the absorberliquid is prevented, provides a particularly high operationalreliability also for permanent operation or standby mode over many yearssince a chemical reaction of the absorber liquid with the reservoirvessel and thus also a chemical dissociation of the absorber liquid iskept extremely low.

Other features which are considered as characteristic for the inventionare set forth in the appended claims.

Although the invention is illustrated and described herein as embodiedin an injection system and an associated operating method, it isnevertheless not intended to be limited to the details shown, sincevarious modifications and structural changes may be made therein withoutdeparting from the spirit of the invention and within the scope andrange of equivalents of the claims.

The construction and method of operation of the invention, however,together with additional objects and advantages thereof will be bestunderstood from the following description of specific embodiments whenread in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEW OF THE DRAWING

FIG. 1 is a diagrammatic, illustration of an injection system for anabsorber liquid according to the invention; and

FIG. 2 is a diagrammatic, illustration of a detail from a boiling-waternuclear power plant with an injection system according to FIG. 1 foremergency shutdown of the reactor core.

DETAILED DESCRIPTION OF THE INVENTION

Identical parts are provided with the same reference symbols in bothfigures. Referring now to the figures of the drawing in detail andfirst, particularly, to FIG. 1 thereof, there is shown an injectionsystem 2 which is used to make available and to inject in an on-demandmanner pressurized absorber liquid 4 into a component, which isconnected downstream of the injection system 2, of a plant, inparticular into a nuclear reactor. In the exemplary embodiment, theabsorber liquid 4 is a 13% sodium pentaborate solution which, in thecase of an incident in a boiling-water reactor (not illustrated furtherhere), is to be introduced into the reactor core, wherein the boronatoms of the boron solution capture free neutrons on account of theircomparatively high absorption cross section for neutrons. In this way,the reactor can be reliably shut down in a relatively short period oftime (approximately 20 seconds after the injection of the absorberliquid 4).

In order to store the absorber liquid 4, a pressure-stable reservoirvessel 6 is provided, which reservoir vessel is completely filled withthe absorber liquid 4 during the storage period. Here, the reservoirvessel 6 is an upright cylindrical tank with a bottom region 8 and aceiling region 10 which are in each case in the shape of a semi-sphere.A structural height H and a diameter D and thus also the volume of thereservoir vessel 6 are matched to the intended use in a nuclear powerplant and have, for example, the values H=7.0 m and D=0.8 m. Thecapacity of the reservoir vessel 6 thus corresponds to the quantity ofabsorber liquid 4 which is provided for the emergency shutdown of thereactor core. A vessel wall 12 is made from a particularlypressure-stable and corrosion-resistant steel wall of a high-gradesteel, for example of an austenitic steel.

In accordance with the configuration of the injection system 2 as apassive safety system, the absorber liquid 4 must be stored permanentlyunder an operating pressure of preferably about 150 bar over the storageperiod which may last many years, that is to say in standby mode, as itwere. The temperature of the absorber liquid 4 here should, however, notsubstantially exceed room temperature in order to avoid increasedreactivity which could lead to corrosion of the surrounding vessel wall12 and to a decomposition of the boron solution. In the exemplaryembodiment, storage of the absorber liquid 4 at a temperature of about30° C. is therefore envisaged.

Because of the comparatively low temperature of the absorber liquid 4,the injection system 2 is configured for a particularly high operationalreliability during the standby mode, wherein furthermore any influenceon the absorber liquid 4 due to the measures necessary to generatepressure should, if possible, be avoided. In order to realize the statedtemperature and pressure conditions in the reservoir vessel, thegeneration and maintenance of a pressure cushion, which are based on theevaporation of a propelling fluid 14 and associated with a strongdevelopment of heat, are therefore decoupled from the reservoir vessel6. To this end, a separate pressure vessel 16 is provided which isconfigured in the exemplary embodiment similarly to the reservoir vessel6 and also has approximately the same dimensions. The pressure vessel 16here therefore has the same structural height H and the same diameterD′=D as the reservoir vessel 6. Both vessels are additionally arrangedat the same height. While the volume of the pressure vessel 16 issubject to boundary conditions which, as far as possible, areprespecified by the intended use (in particular by the operatingpressure to be realized, the quantity of absorber liquid 4 to bedisplaced and, if appropriate, by further design criteria), there is afar-reaching design freedom with respect to the concrete shape andarrangement of the pressure vessel 16 similar to the case of thereservoir vessel 6.

The pressure vessel 16 is filled in the case of operation up to afilling height h with the liquid propelling fluid 14. The propellingfluid 14, in this case water, is heated by a heating device 18, whichcan be regulated, and evaporates partially in the process, with theresult that a vapor cushion 20 forms over the liquid level, in this casetherefore a water vapor cushion, which, owing to its compressibility,effects the actual pressure storage. The regulation of the heatingdevice 18, which is formed for example by electric heating elements orby a heat exchanger system and is preferably arranged in a lower regionof the pressure vessel 16, is effected such that approximately constantoperating pressure of about 150 bar is maintained over the entirestandby time. To this end, provision is made to heat the water locatedin the pressure vessel 16 to an average temperature of about 340° C.These values correspond to the saturated vapor pressure and thesaturated temperature. A vessel wall 12′ of the pressure vessel 16 musttherefore not only be particularly pressure-stable, but alsocomparatively heat-resistant. In order to reduce the heat losses(especially on account of heat radiation), the pressure vessel 16 isprovided on its outside with a thermal insulation (not illustrated infurther detail).

The pressure vessel 16 is, on the medium side, connected via an overflowline 22 to the reservoir vessel 6, by which the same pressure conditionsprevail in the entire injection system 2. Here, the overflow line 22 isguided out of the bottom region 8′ of the pressure vessel 16 in themanner of a siphon. The overflow line 22 therefore has a lower partialsection 23 with a low point which is located below the bottom of thepressure vessel 16. Connected to the lower partial section 23, viewed inthe direction of flow of the propelling fluid 14 (with reference to theinjection process), is a vertical riser 24 which finally merges into asubstantially arched upper partial section 25. The high point of theupper partial section 25 here lies above the ceiling of the reservoirvessel 6. The overflow line 22 is, in the direction of the reservoirvessel 6, guided into a connection flange 26 which projects out of thedome-type ceiling region 10 of the reservoir vessel 6. Due to thepressure exerted by the vapor cushion 20, the liquid propelling fluid 14completely fills the overflow line 22. Any air cushion which may haveoriginally been present in the overflow line was already displacedduring the preceding heating process at approximately 100° C. At aboundary surface 27 between the absorber liquid 4 (boron solution withcomparatively high density) and the propelling fluid 14 (water withlower density), the two liquids do not mix owing to the difference indensities. Rather, a layered liquid column is formed there.

Due to the way the line is guided, a convective transport of heat insidethe overflow line 22 can, if the line diameter is appropriatelydimensioned, be neglected just like the conduction of heat. In otherwords: the liquid propelling fluid 14, which is located in an issueregion 28 to the reservoir vessel 6 or just above in the overflow line22, has approximately the same temperature as the absorber liquid 4inside the reservoir vessel 6, that is to say approximately 30° C. Thetemperature of the fluid in the overflow line 22 rises continuously inthe direction of the pressure vessel 16. The absorber liquid 4 istherefore not heated due to the substantially stationary temperaturedistribution inside the overflow line 22.

If the injection system 2 is activated, a shut-off valve or othershut-off apparatus 30 which has been kept shut up until then is openedso that the pressurized absorber liquid 4 can emerge from an outflowopening 32 arranged in the bottom region 8 of the reservoir vessel 6.Connected to the outflow opening 32 is a connection line 34 to thecomponent which is to be supplied with absorber liquid 4, for example abypass of a reactor core. The shut-off apparatus 30 can, as shown here,be integrated into the connection line 34 or else directly into theoutflow opening 32.

During the injection process, the pressure of the vapor cushion 20,which has previously built up in the ceiling region 10′ of the pressurevessel 16, is relieved and in the process pushes the hot water, which islocated under the cushion and acts as propelling fluid 14, into theoverflow line 22 and then into the reservoir vessel 6. During thisprocess, first the hot water, which is originally in the bottom region8′ of the pressure vessel 16, then the saturated water, which is presentdirectly below the vapor cushion 20, and finally the saturated vaporitself, which forms the vapor cushion 20, flows from the pressure vessel16 into the reservoir vessel 6, with the vessel pressure decreasing atthe same time. When the hot water enters the reservoir vessel 6, itsvessel wall 12 is heated comparatively gently, in any case more gentlythan in the case of a direct entry of hot vapor. This avoids atemperature shock and associated material stresses. Additionally, thevapor pressure does not decrease as quickly as would be the case in adirect condensation of the vapor at the cold vessel wall 12 of thereservoir vessel 6.

The hot propelling fluid 14 flows into the reservoir vessel 6 preferablyin a manner such that a swirling or mixing with the cold absorber liquid4 is avoided and the temperature layering which occurred originally dueto the difference in densities therefore remains. That means that thereis a relatively sharply defined boundary surface 27 between the coldabsorber liquid 4 and the hot propelling fluid 14, which also remainsintact over the course of the inflow process and in the processcontinuously wanders downwards. For this purpose, the issue region 28 ofthe overflow line 22 into the reservoir vessel 6 has a throttle element36, provided with a large number of exit nozzles 35 (arranged forexample on a cylinder outer surface), for suitably influencing the flow.A screening sheet 38 is also arranged in the throttle element 36.

The injection system 2 with the stated dimensions is suited particularlyas a quickly activatable boron injection system in a nuclear powerplant, in particular in a boiling-water nuclear power plant. The twopressure vessels connected via the overflow line 22 (reservoir vessel 6and pressure vessel 16) are in this case comparatively slim and tall, sothat the vessel walls 12, 12′ can be kept thin. In this case, owing tothe quick heating of the reservoir vessel 6 during the injectionprocess, the thermal loads are lower than in the case of a shortervessel with correspondingly greater wall thickness. Volume conditionswhich are different from those mentioned above can be more expedient forother intended uses.

FIG. 2 shows a schematic detail from a boiling-water nuclear power plant39 with an injection system 2 according to FIG. 1. A reactor pressurevessel 42 with a core region 44 is arranged in a containment 40. Thereactor pressure vessel 42 is partially filled with a cooling liquid 46.Above the cooling liquid 46, there is vapor 48 which is conducted via avapor line 50 out of the containment 40 and is guided to a turbine (notillustrated in more detail). Cooled cooling liquid 46 is recycled to thereactor pressure vessel 42 via a line 52. The performance of the nuclearreactor can be regulated by inserting and removing the control rods 54into and out of the core region 44. The control rods 54 are in this casemoved by a drive system 56 which is designed in a redundant manner.

If the incident-free ability to manipulate the control rods 54 is nolonger ensured in the case of a serious incident situation, the nuclearreaction can be interrupted by way of injecting boric acid 58 into thecore region 44 of the boiling-water reactor (so-called poison injectionsystem). The boric acid 58 is stored under high pressure in thereservoir vessel 6 of the emergency injection system 2 according toFIG. 1. The injection system 2 has for this purpose the pressure vessel16, which is connected via the overflow line 22 to the reservoir vessel6 for the boric acid 58 and in which a saturated vapor cushion 62 isproduced by heating of water 60.

The injection system 2 and the associated operating method allowpreferably in boiling-water nuclear power plants and in particular inthe case of incident situations a reliable supply of the absorber liquid4, in particular boric acid, intended for an emergency shutdown to thereactor core, wherein, during the preceding storage period, corrosion ofthe reservoir vessel 6 and/or dissociation of the absorber liquid 4 isavoided.

1. An injection system for a neutron poison for an emergency shutdown ofa nuclear reactor, the injection system comprising: an absorber liquidhaving an absorption cross section that captures free neutrons; apressure vessel containing a propelling fluid being essentially water;an overflow line communicating with a core region of a reactor pressurevessel of a nuclear reactor, the core region being external to saidpressure vessel; a reservoir vessel storing said absorber liquid at roomtemperature, said reservoir vessel connected, via said overflow line, tosaid pressure vessel; and an outflow line connected to said bottomregion of said reservoir vessel; and a shut-off apparatus having aclosed position that closes said outflow line and an open position thatopens said outflow line thereby causing the absorber liquid to be pushedthrough said outflow line by the propelling fluid; said reservoir vesselbeing spatially separated from said pressure vessel, and only saidoverflow line connecting said reservoir vessel to said pressure vesselsuch that pressure is equalized between said reservoir vessel and saidpressure vessel; said pressure vessel formed with a ceiling region and abottom region, said pressure vessel including a heating deviceconfigured to heat and partially evaporate the propelling fluid in orderto form a vapor cushion located in said ceiling region of said pressurevessel, wherein an un-evaporated portion of the propelling fluid definesa liquid portion located below the vapor cushion; said overflow lineconnecting said bottom region of said pressure vessel to said ceilingregion of said reservoir vessel, said overflow line including a riser;and said overflow line including a plurality of nozzles discharging thepropelling fluid near said ceiling region of said reservoir vessel. 2.The injection system according to claim 1, wherein said bottom region ofsaid pressure vessel has an outflow opening formed therein for removalof said absorber liquid.
 3. The injection system according to claim 1,wherein said pressure vessel, said reservoir vessel and said overflowline are pressure-loaded components constructed to operate at a pressureof more than 100 bar.
 4. The injection system according to claim 1,wherein said pressure vessel and said heater cooperate such that saidpropelling fluid is always available at a temperature of more than 300°C.
 5. The injection system according to claim 1, wherein said reservoirvessel is dimensioned to hold a quantity of said absorber liquid whichis sufficient for shutting down the nuclear reactor.
 6. The injectionsystem according to claim 1, wherein said pressure vessel is dimensionedto hold a quantity of said propelling fluid which is sufficient tocompletely displace said absorber liquid from said reservoir vessel. 7.The injection system according to claim 1, wherein said absorber liquidis an aqueous boron solution stored in said reservoir vessel.
 8. Theinjection system according to claim 1, wherein said pressure vessel,said reservoir vessel and said overflow line are pressure-loadedcomponents constructed to operate at a pressure of approximately 150bar.
 9. The injection system according to claim 1, wherein said pressurevessel and said heater cooperate such that said propelling fluid isalways available at a temperature of approximately 340° C.
 10. Theinjection system according to claim 1, wherein said plurality of nozzlesdischarge the propelling fluid such that the propelling fluid isprevented from mixing with the absorber liquid after the propellingfluid exits said plurality of nozzles.